The Cre recombinase is under the control of the Mx1 promoter. This promoter is silent in healthy mice, but can be induced to high levels of transcription by administration of interferon alpha, interferon beta, or synthetic double-stranded RNA. When combined with a mutant carrying a gene that has been flanked by loxP recognition sites, the expression of Cre recombinase causes the flanked gene to be removed. This provides researchers with the capability to induce the "knockout" at any time during development. There was ~1% background recombination seen in mice not treated with interferon. The percent deletion of the targeted gene varied depending on tissue type, presumably due to the amount of interferon-responsive cells present or to the availability of interferon in each organ.
Cre-lox-System, Genetics Research (Mutagenesis and Transgenesis: Cre-lox system)
Further Reading & Data Sheet
This animal model is bred and managed by The Jackson Laboratory and marketed for commercial use by Ascenion.